应用剂量梯度理论确定射线野参数

Determination of beam parameters by dose gradient analysis

目的 应用剂量梯度理论确定多野治疗方案时每个射线野的权重、楔形板角度及方向。方法 剂量梯度理论指出 ,在X(γ)射线野轴线附近区域内 ,平野的剂量梯度的方向与射线野轴线平行 ;楔形野的剂量梯度的方向与射线野轴线的夹角等于射线野的楔形板角度。对于多野治疗方案 ,为保证靶区剂量均匀 ,靶区范围内任意点总的剂量梯度必须为零 ,调整射线野权重或调整权重结合加楔形板的方法可使这一要求得到满足。依据这一理论 ,对于 2个野交角、3个野共面、3个野非共面这些情况 ,推导得到计算射线野权重、楔形板角度及方向的公式。结果 对颅内肿瘤的 2个野和 3个野非共面方案进行了设计 ,计算得到的靶区剂量是均匀的。结论 对于 2个野、3个野共面或非共面照射的治疗计划 ,采用上述方法可以直接确定射线野权重、楔形板角度及方向 ,避免了手工多次调整 ;对于 3个野以上共面或非共面情况 ,提出确定射线野权重。

Objective To determine beam weight, wedge angles and wedge orientations by dose gradient analysis when a treatment plan has more than one photon beam. Methods As proposed by Sonntag and Sherouse, the dose gradient of a single open photon beam can be represented by a vector pointing towards the radiation source and parallel to the beam central axis. The effect of adding a wedge to the beam is to introduce a simple transaxial gradient. The dose gradient due to an ensemble of beams is the weighted sum of the constituent beams' individual gradients. The optimization criterion that dose be homogeneous over an irradiated volume is equivalent to the criterion that the magnitude of the dose gradient be zero throughout that volume. Given a fixed ensemble of beams, there are two ways to render the dose gradient zero: (1) To adjust relative beam weights and (or) (2) To add wedges to some beams. Here formulas are derived for calculating beam weights, wedge angles and collimator angles when a treatment plan has two coplanar beams, three coplanar beams or three noncoplanar beams. Results Two treatment plans are tested. One plan has two noncoplanar beams, and the other has three noncoplanar beams. The resulting dose distribution in the target volume is homogenous. Conclusion Through analysis of dose gradient, treatment plans can be improved as compared to those obtained through conventional manual trial and error process and planning time can be much reduced.